1. Technical Field:
The present invention relates to bone screws and, more particularly, to improvements in guided bone screws whereby safe insertion is facilitated and stability is enhanced.
2. Discussion of the Prior Art:
A prior art bone screw disclosed in U.S. Pat. No. 4,175,555 (Herbert) has like-handed thread at its distal and proximal ends separated by an unthreaded intermediate section. To function as a compression screw the proximal end thread pitch is less than the distal end thread pitch. Thus, when the screw is advanced in pre-tapped bores through two severed bone fragments being joined, the large pitch distal end of the screw advances a greater distance in the remote bone fragment, per screw turn, than the distance advanced by the proximal end of the screw in the near bone fragment. The result is a compression of the bone fragments at the fragment interface.
The Herbert bone screw described above has certain drawbacks that affect its practical utilization. For example, the diameter of the unthreaded intermediate section is smaller than the major diameter of the threaded distal end, and smaller than the minor diameter of the threaded proximal end. As a consequence, the unthreaded intermediate section resides in a bore section that must be large enough to accommodate the minor diameter of the proximal end thread and, therefore, does not fill the bore. The result is a lateral instability that sacrifices the integrity of the fragment interface during the healing process.
The unthreaded intermediate section of the Herbert bone screw is also required to be axially longer than each of the threaded end segments. The stated reason for this requirement is to assure that there is no thread at the fragment interface. While it is necessary to avoid thread at the fragment interface, the elongation of the intermediate screw section in the Herbert screw results in axially short threaded sections (i.e., fewer turns) and, consequently, less available compressive forces at the interface.
In addition, the unthreaded intermediate section of the Herbert bone screw is required to have a diameter that is smaller than both the major and minor diameters of the proximal end thread. This adds to the lateral instability described above. More specifically, the pre-tapped bore in the proximal bore section must be diametrically wider than the smaller diameter intermediate section. As a result, the portion of the intermediate section residing in that bore does not fill the bore.